1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display device, and more particularly, to an organic EL display device having improved efficiency of injecting charges from a cathode to an organic layer.
2. Description of the Related Art
An organic EL display device includes an anode formed on a substrate, a hole transport layer, a light-emitting layer, an electron transport layer and a cathode, which are sequentially stacked on the anode. The hole transport layer, the light-emitting layer and the electron transport layer are organic layers formed from organic compounds.
The organic EL display device having the above-described configuration is driven according to the following description.
When a drive voltage is applied to the anode and the cathode, holes from the anode migrate to the light-emitting layer via the hole transport layer and electrons from the cathode migrate to the light-emitting layer via the electron transport layer. The hole transport layer or the electron transport layer itself may be a light-emitting layer. The electrons and holes are recombined at the interface between the hole transport layer and the emitter layer (or the electron transport layer) to generate excitons. As the excitons are deactivated to a ground state, fluorescent molecules of the light-emitting layer emit light, thereby forming an image.
In the organic EL device, in order to lower a drive voltage of the device and to improve charge balance between electrons and holes, it is necessary to increase the efficiency of injecting electrons from the cathode to an organic layer, such as the electron transport layer.
Methods for improving electron injection include using alkali metals having a low work function metal. Examples of such low work function metals include lithium (Li) or magnesium (Mg), co-evaporating Al-alkali metals, or using Al- or Ag-alkali metal alloys, as disclosed in U.S. Pat. Nos. 5,429,884, 5,059,862, 5,047,687, 4,885,211 and so on.
However, the use of a low work function metal is disadvantageous in view of processing manageability and device stability because low work function metals are not stable and are highly reactive. The use of co-evaporation makes it difficult to substantially control the proportion of alloy forming materials. Also, the use of the alloys decreases reproducibility.
Another method for improving electron injection includes forming an electron injection layer including an inorganic compound such as LiF, CsF, SrO or Li2O between a cathode and an organic layer to a thickness of 5-20 nm, as disclosed in U.S. Pat. Nos. 5,776,622, 5,776,623, 5,937,272 and 5,739,635, and Appl. Phys Lett. 73(1998) 1185).
However, according to the referenced methods, an inorganic material must be processed at a high temperature in forming an electron injection layer, and it is quite difficult to form a thin film having a uniform thickness of 5-20 Å.
As described above, in the conventional organic EL device, various attempts to improve efficiency of injecting electrons from a cathode have been made. However, the conventional organic EL display device cannot provide a satisfactory processing manageability, a low drive voltage, and high luminous efficiency.